Ptilinum

1980, Stuart E. Reynolds, Integration of Behaviour and Physiology in Ecdysis, Michael J. Berridge, J. E. Treherne, Vincent Brian Wigglesworth, Advances in Insect Physiology, Volume 15, page 500, Certainly, the ptilinum, a special eversible sac on the head which is evidently a "hatching" structure, is instrumental in causing the puparium to crack along its line of weakness, a longitudinal line around the anterior end, which meets a circular line extending around the anterior margin of what was the 4th visible segment of the larvae cuticle (Laing, 1935).

As for the structure of the ptilinum itself, Strickland (1953) reported that the ptilina of Conopidae 'are voluminous and represent the most rugged type of ptilinum which we have encountered'.

The suture is of the nature of an extremely narrow slit, along the margins of which the wall of the head is invaginated to form a membranous sac or ptilinum, and the walls of the latter are seen to consist of the same layers as the integument.[…]The purpose of the ptilinum is to thrust off the anterior end of the puparium at a time when the contained imago is ready to emerge and to force the fly through soil, etc. (Fraenkel, 1936).

Push through the ground is accompanied by and partially brought about by alternate eversion and retraction of the blood-filled sac at the anterior end of the head — the ptilinum.[…]When the fly appears above ground, no resistance is felt by the ptilinum, and signals cease to be received by the sensory nerve fibers.